This invention relates to shift timing controls, and more particularly, to neutral to drive shift timing controls. Specifically, this invention relates to the control of the fill time for an on-coming friction device during a neutral to drive shift sequence.
Automatic transmissions utilize fluid operated or hydraulically applied clutches to provide drive couplings between the prime mover and the vehicle wheels. When the vehicle is at a standstill and a neutral to drive, either forward or reverse, is requested by the operator, a clutch is applied which establishes a reaction to ground through the transmission and drive wheels. This occurs when the operator moves the manual shift lever to forward or reverse, expecting a quick and smooth transition. There have been many attempts with varying degrees of success in providing for a quick and smooth transition during a neutral to drive (garage shift).
In at least one prior art system, the hydraulic fluid is directed from a pressure regulator valve to an accumulator selector valve to an accumulator feed orifice, after which, it is directed to an accumulator through a circuit restriction to a clutch apply chamber. The circuit restriction can be either an orifice plate disposed in the valve body or simply the tortuous fluid path the hydraulic fluid must overcome between the accumulator and the clutch apply chamber.
With systems of this type, the operator initiates a drive condition by shifting the manual valve, which controls the direction of fluid to the appropriate clutch. When the hydraulic fluid pressure at the clutch is sufficiently high, the clutch piston will begin to stroke against the return springs until the friction plates are engaged. When this occurs, the pressure of the feed oil will increase rapidly until the accumulator is pressurized sufficiently to begin stroking against the accumulator trim pressure and the accumulator spring.
While the accumulator is stroking, the feed pressure to the clutch will increase at a rate controlled by the amount of fluid flow, the accumulator volume and the accumulator spring rate. To accomplish a smooth garage shift, it is desired to have the accumulator begin stroking at that same instant that the clutch piston is first brought into engagement with the friction plates. This is the point at which the clutch return springs are overcome by the clutch piston.
One drawback of the system is that a high feed pressure and a high fluid flow are required. This can be accomplished if the feed pressure is maintained sufficiently high. However, with this system there is difficulty in controlling the pressure rise in the clutch apply chamber in that the high flow rate causes rapid accumulator fill. To overcome this situation, it is necessary to increase the accumulator volume which requires the use of space within the transmission envelope. This space is not always available.
Another disadvantage with maintaining high feed pressures and flow rates is that the accumulator must begin its stroke at the same instant that the clutch pressure overcomes the return springs. The high flow rate will result in a significant pressure drop in the circuit restriction between the accumulator and the clutch engagement chamber such that the accumulator will begin stroking during the clutch prefill cycle and therefore will not have the advantage of utilizing a total accumulator volume to control the clutch engagement time.
Other control schemes have been proposed, one of which permits bypassing of the accumulator feed orifice during the prefill cycle. However, with these control schemes, it is very difficult to provide an accumulator of a size sufficient so that it does not begin stroking during the high fill flow period and yet begins to stroke at the system pressure which will overcome the return springs. The end results of such control schemes is somewhat erratic in that at times a firm shift will occur and at other times, a long shift time will occur.